1. Field of the Invention
The present invention relates generally to the field of nuclear reactors and in particular to a radioactive debris trap to be installed in the primary outlet plenum of a steam generator for removing fine particles and chips of metal from the primary heat transport system of a nuclear power plant.
2. Description of the Related Art
Referring to FIG. 1, and as described in Steam/its generation and use, 40th Edition, Stultz and Kitto, Eds., Copyright©1992, The Babcock & Wilcox Company, and Steam/its generation and use, 41st Edition, Kitto and Stultz, Eds., Copyright©2005, The Babcock & Wilcox Company, recirculating steam generators (RSGs) used in nuclear power plants are supplied by a number of manufacturers worldwide as part of pressurized water reactor (PWR) or pressurized heavy water reactors (PHWR) (mainly CANDU) systems. These are large devices, ranging in height from approximately 38 to 73 ft (11.6 to 22.3 m) and weighing from approximately 50 to 790 tons (45 to 717 tm) each. Each RSG is a vertical shell, inverted U-tube heat exchanger with steam-water separation equipment located above the tube bundle inside the upper shell (or steam drum). A cylindrical shroud or bundle wrapper surrounds the tube bundle separating it from the lower shell. This creates an annular region which serves as the downcomer to return the recirculated water from the steam separators to the tube bundle inlet at the bottom of the unit. In a feed ring type RSG, generally designated 100 as illustrated in FIG. 1, feedwater is introduced by a nozzle and header to the top of the downcomer, and flows with the separator return flow down and into the tube bundle. In a preheater type RSG, feed flow enters the steam generator through a nozzle and feedwater distribution box to the baffled section at the cold leg outlet end of the tube bundle where it is heated to saturation before joining with the hot leg riser flow within the tube bundle.
The flow configuration and the major design features of a typical feed ring type RSG are as follows. The hot primary coolant enters a portion of the vessel primary head 110, via primary inlet nozzle 120, which is separated into two plenums 130, 140 by a divider plate 50. The primary coolant flows through the inside of the U-tube bundle 150 and exits the steam generator 100 through the primary head outlet plenum 140 and primary outlet nozzle 160. In most RSG designs, the U-tubes make a continuous 180 degree bend at the top of the tube bundle. In the configuration shown, secondary-side feedwater enters the upper shell 170 via feedwater nozzle 180 and is conveyed to a feed ring (not shown) and is mixed with water returning from the steam-water separation equipment 190 located in the upper shell 170. The water flows down the downcomer annulus between the shroud and the shell to the tubesheet where it enters the tube bundle. The secondary-side water is heated as it passes up through the tube bundle generating steam through nucleate boiling heat transfer, creating a two-phase flow. Steam of 10 to 40% quality, depending on hot-side or cold-side U-tube bundle location, exits the tube bundle and is distributed to the primary and secondary steam separation equipment 190 in the upper shell 170 to send effectively moisture-free (<0.25% water) steam to the secondary-side power cycle via steam outlet nozzle 200. Water leaving the steam separators is recirculated down the annulus where it mixes with the feedwater before being returned to the bundle inlet for further steam generation.
During operation, debris can sometimes begin to accumulate in the primary coolant loop or primary heat transport (PHT) system of such steam generators 100. Depending upon the source of the debris, the type of debris which can typically be found in a PHT system can measure 1 square mm or less, or the debris fragments can be as large as 2 mm wide by 4 mm long. Damage and defects caused by debris can cause a problems for nuclear power plants. Thus, it logically follows that various debris trapping devices have been developed in response to the industry wide problems caused by debris.
For example, U.S. Pat. No. 4,684,496 to Wilson et al. (“Wilson”) describes a debris trap for a pressurized water nuclear reactor to be installed into the reactor vessel itself. The debris trap disclosed in Wilson is mounted within a bottom nozzle of a fuel assembly so as to capture and retain debris carried by coolant flowing from the lower core plate openings of a nuclear reactor to a fuel assembly and is made up of a plurality of straps aligned with one another in a crisscross arrangement.
However, due to the large scope of the problems caused by debris in nuclear power plants, there remains a clear need for a simple debris trap which can remove a greater amount of debris and reduce the problems caused by debris.